Fixed the buffer overflow by moving the screen space modulus to where the sprite is drawn.

Makefile

@@ -1,7 +1,7 @@
 all: build
 
 reset:
-	powershell -executionpolicy bypass .\scripts\reset_build.ps1
+	powershell cutionpolicy bypass .\scripts\reset_build.ps1
 
 build:
 	meson compile -j 4 -C builddir
@@ -15,13 +15,13 @@ debug_build:
 	meson compile -j 4 -C builddir
 
 run: build
-	./builddir/b8rk.exe 1 120 ./roms/pong.ch8
+	./builddir/b8rk 1 120 ./roms/pong.ch8
 
 debug: build
-	gdb --nx -x .gdbinit --ex run --args builddir/runtests.exe
+	gdb --nx -x .gdbinit --ex run --args builddir/runtests
 
 debug_run: build
-	gdb --nx -x .gdbinit --batch --ex run --ex bt --ex q --args builddir/b8rk.exe 1 120 ./roms/pong.ch8
+	gdb --nx -x .gdbinit --batch --ex run --ex bt --ex q --args builddir/b8rk 1 120 ./roms/pong.ch8
 
 clean:
 	meson compile --clean -C builddir

src/chip8.cpp

@@ -283,9 +283,8 @@ void Chip8::OP_Dxyn() {
   uint8_t Vy = (opcode & 0x00F0u) >> 4u;
   uint8_t height = opcode & 0x00Fu;
 
-  // wrap if going beyond the screen boundaries
+  uint8_t xPos = registers[Vx];
-  uint8_t xPos = registers[Vx] % CHIP8_WIDTH;
+  uint8_t yPos = registers[Vy];
-  uint8_t yPos = registers[Vy] % CHIP8_HEIGHT;
 
   registers[0xF] = 0;
 
@@ -295,7 +294,11 @@ void Chip8::OP_Dxyn() {
     for(size_t col = 0; col < 8; ++col) {
       uint8_t spritePixel = spriteByte & (0x80u >> col);
 
-      size_t pixel_at = (yPos + row) * CHIP8_WIDTH + (xPos + col);
+      // wrap if going beyond the screen boundaries
+      size_t draw_row = (yPos + row) % CHIP8_HEIGHT;
+      size_t draw_col = (xPos + col) % CHIP8_WIDTH;
+
+      size_t pixel_at = draw_row * CHIP8_WIDTH + draw_col;
 
       uint32_t* screenPixel = &video[pixel_at];